The invention relates to a grid for the absorption of X-rays.
X-ray techniques utilize such grids as anti-scatter grids for the absorption of scattered radiation, arising in the tissue of the patient, before the characteristic X-ray signal caused by the different attenuation properties of the tissue examined is incident on the X-ray detector.
The radiation that is caused by the scattering of the X-ray photons in an object to be examined and cannot be used is shielded by shutters from the detector in order to reduce the scattered radiation component of the overall signal, so that scattered radiation that is incident at an angle is absorbed and does not reach the detector. Diaphragms of this kind are also referred to as anti-scatter grids or anti-scatter lamellae. For customary single line detectors the anti-scatter grids are usually constructed in the form of sheet metal lamellae.
The X-rays emitted by an X-ray source traverse the patient and are attenuated in conformity with the varying density and chemical composition of the tissue or bone to be examined. At the same time scattered radiation is added to the X-ray signal. In order to reduce such scattered radiation that falsifies the primary X-ray image to be formed, the X-rays are made to traverse an anti-scatter grid that is focused onto the focus of the radiation source. It is thus achieved that only the X-ray quanta that are characteristic of the attenuation of the irradiated object are detected.
U.S. Pat. No. 5,099,134 discloses a collimator (anti-scatter grid) and a method of constructing such a collimator. The collimator is formed by a frame which absorbs X-rays and includes an arrangement of first and second partition plates. The partition plates are provided with respective slits that extend in the longitudinal direction of the partition plate and enable the first partition plates to be inserted into the second partition plates at the corresponding angle. At its inner sides the rectangular frame is provided with slits for receiving the respective ends of the partition plates.
The manufacture of such two-dimensional anti-scatter grids is subject to given limits which are imposed by the complexity of the partition plates. The manufacture of anti-scatter grids of large dimensions, for example as used for large-area detectors, has been found to be difficult because bending of the large partition plates interferes with easy and correct engagement of the slits of the partition plates.
Large-area two-dimensional anti-scatter grids are used, for example, in multi-line CT (computed tomography) apparatus. The construction of CT examination apparatus is such that the radiation source is mounted so as to face the detector on a gantry which rotates about the patient while the patient is slowly displaced on a table. Vibrations of the gantry are transferred to the anti-scatter grid and the X-ray detector and have a negative effect on the image quality of the image to be formed. Such negative effects cannot be imitated, so that such effects falsifying the image can be reduced to a limited extent only during image processing at a later instant. The X-ray detector extends in two length dimensions, its dimension in the direction of the gantry being a number of times larger than its dimension in the direction of the longitudinal axis of the patient.
In order to enable fast X-ray operations, the width of the X-ray beam is increased. Consequently, a larger surface of the object to be examined, and hence also a larger volume, is scanned by means of a single scan. As a result, the scattered radiation component increases. In order to reduce such an increasing scattered radiation component, the height of the anti-scatter grid is increased. Known anti-scatter grids, however, do not have the ruggedness required for this purpose.
A further possibility for manufacturing two-dimensional anti-scatter grids with the necessary precision consists in the removal of material from a large block of material. Such manufacturing processes, however, are very expensive and not suitable for the production in large numbers.